Numerous electronic circuits and systems, albeit finding application in very different technical fields, have in common the fact that their function is closed-loop control of the peak of a periodic voltage. Forming part of this category are, for example, certain apparatuses of widespread use in the field of radiofrequency applications; namely:                feedback systems for protection of radiofrequency power amplifiers against “load mismatch”; see, for example: L. La Paglia, A. Scuderi, F. Carrara, and G. Palmisano, “A VSWR-protected silicon bipolar RF power amplifier with soft-slope power control”, IEEE J. Solid-State Circuits, vol. 40, pp. 611-621, March, 2005;        systems for closed-loop control of the output power in radiofrequency transmitters; see, for example: P. Cusinato, “Gain/bandwidth programmable PA control loop for GSM/GPRS quad-band cellular handsets”, IEEE J. Solid-State Circuits, vol. 39, pp. 960-966, June, 2004,        systems for the linearization of radiofrequency transmitters based upon the “envelope feedback” technique; see for example: T. Arthanayake and H. B. Wood, “Linear amplification using envelope feedback”, IEE Electronics Letters, vol. 7, no. 7, pp. 145-146, 1971;        systems for implementation of automatic gain control (AGC) in radiofrequency receivers; see, for example: R. Reimann and H. M. Rein, “A single-chip bipolar AGC amplifier with large dynamic range for optical-fiber receivers operating up to 3 Gbit/s”, IEEE J. Solid-State Circuits, vol. 24, pp. 1744-1748, December, 1989.        
The schematic circuit diagrams from which the aforesaid apparatuses draw inspiration can be represented in the simplified form appearing in FIG. 1.
In general, a variable-gain block 10 constitutes the direct path (variable-gain direct path) of the system. It receives at input a voltage signal in and supplies at output a voltage signal out, where in and out are periodic signals of time. The output out is monitored via a peak detector 20, and the peak detected is compared by an error amplifier 30 with an appropriately set reference voltage 35. The difference signal 40 (gain-control voltage) thus obtained drives the gain-control terminal 15 of the direct path. Thanks to the feedback, in the presence of a sufficiently high loop gain, the steady-state value of the output peak tends to equal the reference value, which is the ultimate target of the systems under examination.
The dashed part of the closed loop in FIG. 1, designated by the reference number 50, identifies the feedback path.
In the more general case where control is required of a linear combination of a number of periodic voltage signals instead of just one signal, the function implemented by the feedback path can be expressed in an analytical form as follows:
                              V          o                =                  G          [                                    max              ⁢                              {                                                      ∑                    k                                    ⁢                                                            A                      k                                        ⁢                                                                  v                        k                                            ⁡                                              (                        t                        )                                                                                            }                                      -                          V              REF                                ]                                    (        1        )            
where VO is the output voltage of the feedback path (designated by the reference number 40 in FIG. 1), Vk(t) are the periodic voltages to be controlled, Ak are the coefficients with which the inputs are linearly combined, VREF is the reference voltage, and G is the comparison gain. The sign of G is chosen in such a way as to ensure a negative feedback, and generally |G|>>1. The function max{·} defines the maximum in time of the argument periodic function.
The solutions of the known art for implementation of the function described by Eq. 1 reflect the schematic circuit diagram illustrated in FIG. 2. The signal is processed entirely in voltage through a signal-conditioning block 60, which performs the combination of the inputs, a voltage peak detector 70, which extracts the peak of the combination, and a voltage comparator 80, which compares the peak with the reference voltage VREF.
For the systems in question the requirement of bandwidth is usually essential. For example, in feedback systems for the protection of radiofrequency power amplifiers against load mismatch, maximizing the bandwidth means minimizing the times of response to the overvoltage stress on the final stage. However, such systems, for the very fact that they are feedback systems, require an appropriate compensation to ensure a congruous phase margin in the frequency response.
In general, it may be stated that obtaining a wide closed-loop bandwidth for a fixed phase margin is a task that is the more arduous, the greater the number of loop-gain poles. This is the reason why the solution illustrated in FIG. 2 is limited. It entails, in fact, the presence of at least two low-frequency poles: one is the pole through which the loop compensation is performed and is inherent in the voltage comparator (normally implemented as a normal operational amplifier); the other is the pole introduced by the hold capacitor on which the peak detector is based.
In the context of voltage-peak detectors, the most classic solutions of embodiment are those illustrated in FIGS. 3A, 3B and 3C.
In particular, FIGS. 3A and 3B show the open-loop solutions based upon the use, as rectifier element, respectively of a diode D1 (see for reference: J. Millman and A. Grabel, “Microelettronica”, Milan, McGrawHill, 1994, pp. 59-61) and a transistor T1 (see for reference: R. G. Meyer, “Low-power monolithic RF peak detector analysis”, IEEE J. Solid-State Circuits, vol. 30, pp. 65-67, January, 1995).
The advantage of the solution that uses the transistor as compared to the one that employs the diode lies in a more contained requirement in terms of current of the input signal thanks to the gain βF of the transistor T1, to the advantage of the current consumption of the stage upstream. Both of the open-loop solutions, however, suffer from problems of accuracy caused by the fact that the p-n junction responsible for rectification presents a non-zero voltage drop between the input and the output. The error that follows therefrom can be corrected a posteriori, but the effectiveness of the correction is limited since said voltage difference depends upon the crest factor of the input waveform.
In order to improve the accuracy, it is possible to resort to feedback peak detectors, such as the one shown in FIG. 3C. In the presence of a high loop gain, the difference between the peak of the input voltage and the output voltage tends to zero. However, the solution is practicable only at relatively low frequencies, given the presence of an additional delay of propagation due to the gain stage.